Avoid surprises, know your 500mb charts…

The calm after the bomb… cyclone

The photo above was taken after witnessing the last significant bomb cyclone to hit our area.

On Nov 26 2019 a rapidly decreasing “bomb” low produced a bomb cyclone that affected Southern Oregon and Northern California.

This bomb cyclone produced a wave of over 75 ft at the Scripps Buoy 168. The largest wave ever recorded in the Eastern Pacific.

It occurred a depth of 1132 ft(eg a surface wave without significant upwelling). This is also a buoy that delivery captains are very familiar with. So this wave was an eye opener.

This was the first storm I had ever really watched while monitoring the effects of the upper atmosphere on a surface low, and it was a learning experience.

We had driven down to the Southern Oregon coast for a Thanksgiving family event. As we were driving down I was watching the surface and 500mb charts on the OPC website.

I had been reading the Chen-Chesneau book on upper atmosphere in route design, and this storm was on the leading edge of a negative trough, and was intensifying.

I can’t recommend strongly enough that you take a read through their book “Heavy Weather Avoidance and Route Design”.

This book is a definitive guide on how to use the 500 mb charts to better plan multi day routes.

I used to wonder why the 500mb was important if you already have a text forecast and surface analysis in hand?

When looking at weather windows greater than a few days, the 500MB chart is like a roadmap for upcoming weather.

Since a number of us are racing in Pacific races every few years such as Transpac, Pacific Cup, Vic-Maui, and since these races take 14-28 days to complete, knowing how to identify problem areas based on the 500mb chart is very helpful.

One important thing discussed in the Lee-Chesneau book are the configurations of blocking highs. The NWS publishes an excellent primer document on the upper atmosphere, and we have used the images from document for this blog post.

If you’d like the read that primer, the NWS publishes the jet-stream basics document here.

There are two blocks that are predominant in the Eastern North Pacific.

The most common of which is the rex block, this block is characterized by an upper level high with an upper level low directly south of it:

Example rex block

The more stable configuration is called an omega block. In an omega block there are lows at the east and west edges on the south of the high. This indicates that the forecast at the surface is mostly stable.

I recently saw a blog post referring to a surface high as an omega high because it was “round.” Two notes on this which are important to understand are

1. an omega high is an upper atmosphere(500mb) high, not a surface high, it doesn’t show up on a surface chart

2. a significantly round surface high is not as indicative of long term stability as an omega block.

The image below is an image of an omega block in the upper atmosphere:

Example omega block

One of other thing to understand is the effects of different types of troughs and ridges on the surface chart.

In the case of a trough that is aligned from southwest to northeast, aka a positive tilted trough, lows are relatively stable. If the UPPER level wind speeds(not surface level speeds) on the leading side of the trough are faster than the trailing side of the trough, lows in front of the trough tend to dissipate.

But in the case where the trough is oriented from the Northwest towards the Southeast, storms on the leading side of the axis can intensify as seen below.

This is a configuration that can indicate rapid deepening of the low(think Queens Birthday storm, Thanksgiving week storm discussed above, and Fastnet), make note of these leading edge surface lows and monitor them frequently:

Example of a negative trough, note the storms east of axis

With knowledge of what weather may be expected to set up based on upper atmosphere information, you can at least gain some intuition about how the weather will progress beyond the current forecast. This can help in determining when and what to monitor while offshore.

Hope all is well,
RtW